Continuous weight loss blenders of various types have been used for supplying a blend or mixture of different pelletized, free-flowing plastic material ingredients to a plastic extrusion machine. One type of continuous weight loss blender includes a weight metering component for each material ingredient to be blended. Each ingredient is weighed as it is discharged by a driven auger from an individual weigh hopper to a common mixing hopper where the ingredients are mixed or blended. From the mixing hopper, the blend or mixture of the different plastic ingredients is supplied to an extrusion machine typically located beneath the mixing hopper.
In operation of continuous weight loss blenders, the weigh hoppers are initially filled with a given weight of each plastic material by a gate valve, squeeze valve, or directly from a vacuum receiver. As each material is fed from its weigh hopper by a feed auger, a load cell senses the loss in weight of each hopper and generates analog signals representative of the weight loss. The analog signals are converted to digital signals and provided to a digital computer control microprocessor that calculates the material discharge rate from each hopper and adjusts the speed of the associated auger to provide the required output of each ingredient to satisfy the preset ratio or proportion of the ingredients in the blend to be supplied to the extrusion machine.
As each weigh hopper empties, it must be refilled. In particular, as a preset low material level of each weigh hopper is reached, a signal is generated to a refill device to supply plastic material to the hopper to a preset high material level. During the refill period, the weight loss metering system of the blender is actually in volumetric operation, requiring the computer control microprocessor to calculate how much material was emptied from the hopper during the refill period. As a result, considerable effort has been directed toward developing systems for controlling the blender operation during the refill period in order to improve the accuracy of the recipe (i.e., proportions or ratios of ingredients) provided by the blender to the extrusion machine.
One control system adopted to improve blender accuracy employs two capacitance proximity level switches in the lower mixing hopper to increase or decrease the speeds of each motor-driven auger to achieve the preset ratio of ingredients and continuous operation. The augers are operated at speeds to maintain a material level in the mixing hopper between the proximity switches. If the high level switch is reached by the material in the mixing hopper, the speed of the augers is reduced. On the other hand, if the low level switch is reached, the speed of the augers is increased. This type of operation is not suitable for direct extrusion since a sufficiently accurate discharge rate of the blended materials to the extrusion machine cannot be established and maintained for a given machine throughput.
Another control system adapted to improve blender accuracy uses a single capacitance level switch that monitors the level of ingredients in the mixing hopper and adjusts the speed of the motor-driven augers of the respective weigh hoppers to maintain an equilibrium material level in the mixing hopper. Since the capacitance switch is actually a volumetric device and only monitors the material level in the mixing hopper, any variations in the bulk density of the ingredients will adversely affect the accuracy of the material discharge rate to the extrusion machine. For example, the weight of material in the mixing hopper will be lower as the bulk density of the ingredients decreases and will be higher as the bulk density increases. Thus, although the feed rate of the materials to the mixing hopper is accurately known, the discharge rate to the extrusion machine is not known with accuracy since the actual weight of the ingredients in the mixing hopper is not known with accuracy. Inaccurate material discharge rates to the extrusion machine thereby result.
Still another control system adapted to improve blender accuracy involves a dosing technique wherein a central, gravity-feed weigh hopper discharges the major material ingredient directly to an underlying mixing hopper and secondary auger fed weigh hoppers are provided for the concurrent discharge of minor material ingredients to the mixing hopper. The computer control microprocessor of such a blender adjusts the material discharge rate of the secondary hoppers in response to the material discharge rate of the central weigh hopper so as to dose in the correct rate of the minor material ingredients to maintain the preset ratio of material ingredients in the blend. Unfortunately, as the discharge rate of each weigh hopper is altered to add material, the discharge rate of the central weigh hopper is, in effect, changed. Thus, as the material discharge rates of the secondary hoppers are adjusted, the ratio of material ingredients varies from the preset specification therefor such that the microprocessor continually tries to modify operation of the blender to stabilize the ratio within the preset limits. Moreover, this control system is affected by variations in the bulk density of the ingredients and requires extended time periods to adjust thereto.
It is an object of the present invention to provide an improved apparatus and method for metering different material ingredients for supply to a material processing machine, such as an extruder, that overcomes the aforementioned deficiencies and disadvantages.
It is another object of the present invention to provide an improved apparatus and method for metering different material ingredients for supply to a material processing machine wherein metering of the ingredients is controlled in dependence on a learned material processing rate of the processing machine to maintain a highly accurate ratio or proportions thereof in the blend at the learned material processing rate.
It is another object of the present invention to provide an improved apparatus and method for metering different material ingredients for supply to a material processing machine wherein metering of the ingredients is controlled in dependence on a learned material processing rate of the processing machine to maintain a substantially constant weight or head of the blend of the ingredients for supply to the machine.